Question:
Why am I seeing lots of "Discarding Renew ... not our server identifier" messages in logs after a server hardware (or OS) upgrade?
Example:
Answer:
The DHCPv6 server DUID by default is "LLT": link layer address and time. While the address should stay the same barring any hardware changes, the four-byte timestamp changes every second. Note from the sample above:
The server DUID is built the first time the server runs and is then stored in the lease file, so if you delete your lease file you get a new one. If you keep your lease file intact, you keep using the old one. Above we see the client presenting a "SERVERID" associated with its previous lease, while the server DUID differs in the four-byte timestamp and the six-byte MAC (link layer) address is the same.
So most likely here, the server has lost its lease file. This is a good example of why the lease file should be maintained in permanent storage, e.g., a hard drive. If your OS deleted it in an upgrade, that could be a bug in its upgrade procedure.
Without the lease file, every existing lease will fail to renew, even if the client-provided SERVERID does happen to match the server DUID.
In due time the client will give up on its renewal attempts and will solicit a new lease. The problem will go away eventually, but likely at the cost of most clients having changed IP addresses.